The problem is that the forum is trying to send a mail to someone to notify him/her about a new post (=your post), but failed as the mailserver didn't accept the mail (via SMTP). This can range from "no mailserver is running at all" all the way to "the mailserver is running, but your message smells like spam" and is probably somewhere in the middle of "the mailserver should send those mails, but not bad stuff, and all this in a secure way" (= the typical triangle. Pick two.)

We have this problem for a few weeks now, "the admin is looking into it" is the current state as far as I know.
(same for this silly double login thing, for the record)

_________________MfG. DonKult
"I never make stupid mistakes. Only very, very clever ones." ~ The Doctor